It is known to maintain a laminar flow along an aircraft surface such as a wing surface by suction through apertures for influencing the boundary layer. The suction may be so-controlled that an artificial laminar flow is enforced and a turbulent flow is substantially avoided or at least delayed. Reference is made in this connection to an article entitled "Second Skin" (Zweite Haut) by H. L. Hillebrand in Flug Revue, December 1985, pages 68 to 70 describing two efforts for controlling boundary layer flow.
The so-called "Lockheed structure" involves a sandwich construction of a wing portion formed of honeycomb sheets covered with a graphite epoxy skin which in turn is covered by a further thin skin provided with longitudinal slots extending cross-wise to the flow direction on the upper and lower side of the wing near the wing leading edge. Suction is produced inside of the wing chambers and air is sucked into the wing through the longitudinal slots. The extent of the suction is controlled by a control unit.
The so-called "Douglas structure" includes a perforated thin sheet metal layer of titanium applied to a fiberglass support structure. The perforations include a multitude of very small apertures which are closely spaced to each other. These apertures are in the wing and suction is applied to remove boundary layer air through these apertures.
It is also known from European Patent Publication EP 0,436,243 A2 (Rudolph et al.), published on Jul. 10, 1991, to use an anti-icing system of an aircraft also for laminar flow control. The aperture through which hot air is blown out can also be used for sucking air through the apertures into the wing for laminar flow control. The flow is reversed in the anti-icing mode or vice versa. Rudolph et al. do not provide an exact control of the air quantity to be sucked off nor of the air quantity to be blown out. Thus, neither the blowing operation nor the sucking operation can be adapted to the instantaneously prevailing flow conditions at the leading edge of the wing. The Rudolph system is not capable of avoiding boundary layer separation altogether because in the suction mode it is possible that air may flow out of the apertures from those chambers in which the suction is not applied and the pressure build up in any chamber may result in a chamber pressure larger than the outside pressure. The prior art does not deal with boundary layer flow control in a rudder assembly.
Reference is also made to Aeronautical Engineering, Jan. 27, 1937, pages 98 to 100 where an article entitled "Sucking off the Boundary Layer" by B. S. Shenstone describes original efforts for boundary layer control. Further reference is made to U.S. Pat. No. 5,141,182 (Coffinberry), issued Aug. 25, 1992; U.S. Pat. No. 5,125,597 (Coffinberry), issued Jun. 30, 1992; and U.S. Pat. No. 5,143,329 (Coffinberry), issued Sep. 1, 1992.
The above prior art leaves room for improvement.